1. Field of the Invention
The present invention relates generally to the area of novel strategies for the improvement of chemotherapeutic intervention. In other aspects, the present invention provides novel methods and compositions that combine the potency of DNA damaging agents with the combined delivery of a tumor suppressor. The combination of DNA damaging factors with the heterologous expression of a tumor suppressor gene lead to a pronounced synergy over and above the actions of the individual components.
2. Description of Related Art
Current treatment methods for cancer, including radiation therapy surgery, and chemotherapy, are known to have limited effectiveness. Lung cancer alone kills more than 140,000 people annually in the United States. Recently, age-adjusted mortality from lung cancer has surpassed that from breast cancer in women. Although implementation of smoking-reduction programs has decreased the prevalence of smoking, lung cancer mortality rates will remain high well into the 21st century. The rational development of new therapies for lung cancer will depend on an understanding of the biology of lung cancer at the molecular level.
It is now well established that a variety of cancers are caused, at least in part, by genetic abnormalities that result in either the over expression of one or more genes, or the expression of an abnormal or mutant gene or genes. For example, in many cases, the expression of oncogenes is known to result in the development of cancer. “Oncogenes” are genetically altered genes whose mutated expression product somehow disrupts normal cellular function or control (Spandidos et al., 1989).
Most oncogenes studied to date have been found to be “activated” as the result of a mutation, often a point mutation, in the coding region of a normal cellular gene, i.e., a “proto-oncogene”, that results in amino acid substitutions in the expressed protein product. This altered expression product exhibits an abnormal biological function that takes part in the neoplastic process (Travali et al., 1990). The underlying mutations can arise by various means, such as by chemical mutagenesis or ionizing radiation. A number of oncogenes and oncogene families, including ras, myc, neu, raf, erb, src, fms, jun and abl, have now been identified and characterized to varying degrees (Travali et al., 1990; Bishop, 1987).
During normal cell growth, it is thought that growth-promoting proto-oncogenes are counterbalanced by growth-constraining tumor suppressor genes. Several factors may contribute to an imbalance in these two forces, leading to the neoplastic state. One such factor is mutations in tumor suppressor genes (Weinberg, 1991).
An important tumor suppressor gene is the gene encoding the cellular protein, p53, which is a 53 kD nuclear phosphoprotein that controls cell proliferation. Mutations to the p53 gene and allele loss on chromosome 17p, where this gene is located are among the most frequent alterations identified in human malignancies. The p53 protein is highly conserved through evolution and is expressed in most normal tissues. Wild-type p53 has been shown to be involved in control of the cell cycle (Mercer, 1992), transcriptional regulation (Fields et al., 1990, and Mietz et al., 1992), DNA replication (Wilcock and Lane, 1991 and Bargonetti et al., 1991), and induction of apoptosis (Yonish-Rouach et al., 1991, and, Shaw et al., 1992).
Various mutant p53 alleles are known in which a single base substitution results in the synthesis of proteins that have quite different growth regulatory properties and ultimately, lead to malignancies (Hollstein et al., 1991). In fact, the p53 gene has been found to be the most frequently mutated gene in common human cancers (Hollstein et al., 1991; Weinberg, 1991), and is particularly associated with those cancers linked to cigarette smoke (Hollstein et al., 1991; Zakut-Houri et al., 1985). The overexpression of p53 in breast tumors has also been documented (Casey et al., 1991).
One of the most challenging aspects of gene therapy for cancer relates to utilization of tumor suppressor genes, such as p53. It has been reported that transfection of wild-type p53 into certain types of breast and lung cancer cells can restore, growth suppression control in cell lines (Casey et al., 1991; Takahasi et al., 1992). Although DNA transfection is not a viable means for introducing DNA into patients' cells, these results serve to demonstrate that supplying wild type p53 to cancer cells having a mutated p53 gene may be an effective treatment method if an improved means for delivering the p53 gene could be developed.
Gene delivery systems applicable to gene therapy for tumor suppression are currently being investigated and developed. Virus-based gene transfer vehicles are of particular interest because of the efficiency of viruses in infecting actual living cells, a process in which the viral genetic material itself is transferred. Some progress has been made in this regard as, for example, in the generation of retroviral vectors engineered to deliver a variety of genes. However, major problems are associated with using retroviral vectors for gene therapy since their infectivity depends on the availability of retroviral receptors on the target cells, they are difficult to concentrate and purify, and they only integrate efficiently into replicating cells.
Tumor cell resistance to chemotherapeutic drugs represents a major problem in clinical oncology. NSCLC accounts for at least 80% of the cases of lung cancer; patients with NSCLC are, however, generally unresponsive to chemotherapy (Doyle, 1993). One goal of current cancer research is to find ways to improve the efficacy of gene replacement therapy for cancer by investigating interaction between the gene product and chemotherapeutic drugs. The herpes simplex-thymidine kinase (HS-tK) gene, when delivered to brain tumors by a retroviral vector system, successfully induced susceptibility to the antiviral agent ganciclovir (Culver, et al., 1992). The HS-tK gene product is an exogenous viral enzyme, whereas the wt-p53 protein is expressed in normal tissues, suggesting that the modulation of chemoresistance by alterations in wt-p53 expression might be an alternative approach using a pathway mediated by an endogenous genetic program.
An adenovirus system has potential advantages for gene delivery in vivo, such as ease of producing high titer virus, high infection efficiency, and infectivity for many types of cells. The stability and duration of expression of the introduced gene are still controversial, however. The increase in p53 levels in cells that are sensitive to chemotherapeutic drugs can occur within 6 hours after DNA-damaging stimuli (Fritsche, et al., 1993, Zhan, et al., 1993), although increased p53 DNA binding activity can be reversed over the course of 4 hours if the stimulus is removed (Tishler, et al., 1993). Therefore, a high level of p53 expression can be maintained even after cessation of drug exposure. The expression of wt-p53. protein by Ad-p53 peaks at postinfection day 3 (14-fold greater than endogenous wildtype) and decreases to a low level by day 9 (Zhang, et al., 1993). This suggests that a transiently high level of wt-p53 expression is sufficient to initiate the cytotoxic program in the cancer cell.
p53 has an important role as a determinant of chemosensitivity in human lung cancer cells. A variety of treatment protocols, including surgery, chemotherapy, and radiotherapy, have been tried for human NSCLC, but the long-term survival rate remains unsatisfactory. What is needed is a combination therapy that is used alone or as an effective adjuvant treatment to prevent local recurrence following primary tumor resection or as a treatment that could be given by intralesional injections in drug-resistant primary, metastatic, or locally recurrent lung cancer. Compositions and methods are also needed to developed, explore and improve clinical applicability of novel compositions for the treatment of cancer. Furthermore these methods and compositions must prove their value in an in vivo setting.